This invention relates to a total hip replacement femoral component.
Total hip replacements conventionally use a long intramedullary stem passing the marrow cavity of the bone. These are very successful but when they fail the long stem can create considerable damage inside the bone. It can cause a fracture through the bone around the level of the tip of the stem. The implant can wobble about inside the bone causing the intramedullary cavity to become larger and larger. Revision of failures is always difficult for a surgeon.
Surgeons have always called for a more conservative device. The Smith-Peterson cups of the 1920's and 1930's were merely free floating substantially hemispherical shells placed between the top of the femur and the acetabulum. Apart from paring away the cartilage, there was no bone removal. They were not very successful and the constant movement of bone against metal as the joint moved eventually caused severe bony erosion. In the mid-1940's Judet in France designed a prosthesis whereby the majority of the femoral head was removed and a replacement device was fitted with a peg or nail which passed a short way down the femoral neck. These lasted a little longer than the Smith-Peterson cups, but not much. Small movement of the device against the bone caused friction of the bone and the bending loads on the peg often caused them to break out underneath the bony femoral neck. In the mid-1970's, double cup arthroplasty was tried. There were several designs: Wagner in Germany, an Italian Group, Imperial College and London Hospital (ICLH) and the Tharies design from Amstutz in California. These all removed a fair proportion of the femoral bearing surface by turning it down to a cylindrical form or hemispherical form. A metal shell was then fixed with bone cement on the remaining bony peg. The acetabular cup was quite conventional. Unlike normal total hips, however, which have standard femoral head sizes of 22 mm, 26 mm, 28 mm and 32 mm, these double cup arthroplasties have to have large bearing surface diameters closer to the original hip, typically 40 mm or 41 mm for a small size, 45 mm or 46 mm for a medium size and 50 mm or 51 mm for a large size. These latter double cup designs commonly failed either by a crack progressing around the bone cement between the prosthetic femoral shell and the bone or by a fracture of the bone across from one side of the prosthetic femoral component rim to the other.